Structural effects on adsorption of atmospheric gases in mixed Li,Ag-X-Zeolite

Silver is known to strongly affect the adsorptive properties of zeolites. In the synthesis of mixed Li, Ag low-silica X-type (LSX) zeolite, adding very small amounts of silver and specific dehydration conditions results in a sorbent with enhanced adsorptive characteristics for air separation. The location of the extraframework silver in relation to the aluminosilicate framework is of primary importance for elucidating the effect of silver cations on the adsorptive characteristics of the zeolite. In this work, mixed Li, Ag ion-exchanged zeolites were synthesized and treated to promote the formation of intracrystalline silver clusters. These samples were structurally characterized using Rietveld refinement of neutron powder diffraction data. Structural characterization revealed the presence of cations in a novel site II* in mixed Li,Ag-LSX zeolites that were vacuum-dehydrated at 450°C. Cations in this site II* are more interactive with the atmospheric sorbates of interest than silver at the conventional site II location. Vacuum dehydration at 450°C induced thermal migration of Ag⁺ from site II to site II* and gives rise to the superior properties for air separation.